With the recent trend toward size reduction in electronic appliances, the desire for secondary batteries having a high capacity is growing. In particular, nonaqueous secondary batteries having a higher energy density and better high-current charge/discharge characteristics than nickel-cadmium batteries and nickel-hydrogen batteries have come to attract attention.
Graphite is frequently used as a negative-electrode material for nonaqueous secondary batteries from the standpoints of cost, durability, and capacity. However, there has been a problem that when an active-material layer containing a negative-electrode material and formed on an electrode is densified in order to increase capacity, this densification results in an increased charge/discharge irreversible capacity in initial cycling.
Patent document 1 describes a process for producing a carbon composite material which includes kneading a natural graphite together with a binder to obtain a mixture, primarily heating the mixture at 700-1,500° C. to carbonize the binder, and then secondarily heating the mixture at 2,400-3,000° C. to thereby purify the natural graphite and simultaneously graphitize the carbonized binder. However, the technique disclosed in patent document 1, which is intended to use a flaky or scale-like natural graphite as a starting material, has been insufficient in the densification of an active-material layer and in reduction in charge/discharge irreversible capacity.
Patent Document 1: JP-A-2000-086343
It is expected that the densification of an active-material layer containing a negative-electrode material eliminates most of the interstices present in the electrode, resulting in poor infiltration of an electrolytic solution thereinto. It is thought that the poor electrolytic-solution infiltration renders the migration of lithium ions within the electrode not smooth to cause a deficiency in the electrolytic solution and an uneven potential distribution, leading to a decrease in charge/discharge high-load characteristics.